Apparatus and method for inhibiting propagation of a flame front

ABSTRACT

Apparatus and a method are provided for inhibiting the propagation of a flame front that is ignited by a pumping mechanism which draws a waste stream from a process chamber. The apparatus comprises a foreline for conveying the waste stream which is drawn from the process chamber to the pumping mechanism. The foreline comprises an isolation valve for selectively isolating the pumping mechanism from the process chamber and a bypass positioned around the isolation valve. The apparatus further comprises a controller for actuating the isolation valve. The controller is configured to cause the isolation valve to be closed when the waste stream is initially drawn from the process chamber. During which time the waste stream is conveyed to the pumping mechanism via the bypass, the bypass comprises means for inhibiting propagation of a flame front therethrough. The controller is also configured to cause the isolation valve to be opened once a pressure within a region upstream of the isolation valve has been reduced below a value at which propagation of a flame front within the waste stream can be sustained.

This invention relates to apparatus for inhibiting the propagation of aflame front ignited by a pumping mechanism drawing a waste stream from aprocess chamber following a restart of the pumping mechanism.

As semiconductor processes become increasingly sophisticated, the fluidsused in these processes are becoming increasingly aggressive. There isan increasing risk associated with these processes that the atmospherewithin a vacuum pump used to evacuate the process chamber may comprisepockets of flammable gas or, in the extreme, may be entirely flammable.Conventionally, vacuum pumps have not been designed with suchenvironments in mind. A vacuum pumping mechanism typically comprises ametal rotor cooperating with a metal stator to convey fluid from aninlet of the vacuum pump to an outlet thereof. These components of thepumping mechanism are required to have a close tolerance so that fluidbeing pumped is inhibited from leaking back towards the inlet of thepump. The proximity of these two metal components is, by its verynature, inclined to represent an ignition source as any clashing ofcomponents may generate a spark. Given the aggressive nature of theprocesses being undertaken by these pumps, deformation of the metalcomponents (through corrosion) is increasingly likely, whereby thesetolerances may be significantly reduced. Furthermore, the reactions ofmaterials used in semiconductor processes often lead to a deposition ofmaterials on the surfaces of the rotor and the stator. These depositsfurther reduce the clearances such that the alignment of the componentsof the pumping mechanism may be affected and clashing of the metalcomponents may result. In addition, the deposits formed on the surfacesof the rotors and the stator may become an ignition source, for exampleif they are heated by friction resulting from the increased contact dueto the reduced clearances.

In the event that a flammable atmosphere comes into contact with anignition source an explosion may result. If this explosion leads todamage of the apparatus safety issues are likely to be raised. Acatastrophic breach of integrity may cause projectiles to be formed fromthe components of the pump, creating a hazardous environment to anyother equipment in the vicinity and ultimately to any personnel locatedin the area. If such a breach is less abrupt, leakage of flammable gasmay occur into the environment surrounding the apparatus, and so iffurther ignition sources are available in this area, there may be a riskof further explosion. Depending on the extent of any damage caused bythe explosion, the entire pumping arrangement may need to be taken outof service to permit maintenance to be undertaken. The down time for theoverall process system associated with this unplanned maintenancetypically results in a loss of production.

As discussed above, during operation a vacuum pumping mechanism mayprovide an ignition source for a flammable gas mixture. Consequently, inthe event that the pumping mechanism of a vacuum pump connected to aprocess chamber becomes engulfed in a flammable gas mixture prior toinitiation of operation of the vacuum pump, it is possible that thesubsequent motion of the pumping mechanism could result in an explosion.Such an explosion could propagate back through the inlet of the pumptowards the process chamber.

It is an aim of at least the preferred embodiments of the presentinvention to minimise the hazardous potential of such an explosion.

In a first aspect the present invention provides apparatus forinhibiting the propagation of a flame front ignited by a pumpingmechanism drawing a waste stream from a process chamber, the apparatuscomprising:

-   -   a foreline for conveying the waste stream drawn from the process        chamber to the pumping mechanism, the foreline comprising an        isolation valve for selectively isolating the pumping mechanism        from the process chamber and a bypass around the isolation        valve; and    -   a controller for actuating the isolation valve, wherein the        controller is configured to:    -   cause the isolation valve to be closed when the waste stream is        initially drawn from the process chamber whereby the waste        stream is conveyed to the pumping mechanism via the bypass, the        bypass comprising means for inhibiting propagation of a flame        front therethrough; and    -   cause the isolation valve to be opened when a pressure within a        region upstream of the isolation valve has been reduced below a        value at which propagation of a flame front within the waste        stream can be sustained.

During the initial stage of evacuation, the waste stream is drawnthrough the bypass. As the bypass comprises inhibiting means forinhibiting propagation of a flame front, there is a reduced risk of anydeflagration propagating back towards the process chamber. Theinhibiting means causes a pressure within the region between the bypassand the pumping mechanism to be below a value at which the propagationof a flame front within the waste stream can be sustained.

Evacuation through the bypass, and consequently through the inhibitingmeans, is significantly slower than when the waste stream is conveyeddirectly through the foreline when the isolation valve is open, due tothe increased level of obstructions experienced by the waste stream.Moreover, processes producing flammable gas mixtures are often “dirty”processes that result in high levels of materials being deposited on anysurfaces downstream of the process chamber. For each of these reasons itis desirable to minimise the duration of the passage of the waste streamthrough the bypass. Therefore, once the pressure in the apparatusupstream of the isolation valve has reduced below a value at whichpropagation of a flame front within the waste stream can be sustained,the isolation valve is opened so the waste stream may pass directly fromthe process chamber to the pumping mechanism without necessarily passingthrough the bypass. Propagation of a flame front is now inhibited by thefact that the pressure upstream of a potential ignition source, forexample the pumping mechanism of the vacuum pump, is reduced below avalue whereby propagation of a flame front could be sustained.

The inhibiting means may be provided as a separate component ormechanism located within the bypass. For example, the inhibiting meansmay comprise a flame arrester element and/or valve. If a valve is used,it may be a valve having a variable restriction element for effecting avariable restriction to the flow of a waste stream therethrough.However, the inhibiting means may be provided by altering theconfiguration of the bypass itself. For example, the dimensions or thetrajectory of the bypass may be designed to inhibit propagation of anyincident flame front. The bypass may be configured to have an internalcross sectional dimension smaller than that of the foreline to restrictflow of the waste stream therethrough. Alternatively, or additionally,the bypass may be configured such that it comprises one or moreconvolutions. The inhibiting means may be provided by forming arestriction to the flow within the bypass using either a valve or byproviding a bypass with a smaller diameter bore. For example, for avacuum pump with a pumping speed of 1000 m³/hr may use a bypass havingan open area of approximately 7 mm². The isolation valve may comprise agate valve.

If the inhibiting means is provided by valve means, the controller maybe configured to cause the valve means to be closed prior to operationof the pumping mechanism, and to be opened when a pressure within aregion between the valve means and the pumping mechanism has beenreduced below a value at which propagation of a flame front within thewaste stream can be sustained. The controller may be configured toprevent initiation of operation of the pumping mechanism unless thevalve means is closed.

A second aspect of the present invention provides a vacuum pumpingarrangement comprising a vacuum pump and any of the aforementionedapparatus for inhibiting the propagation of a flame front ignited by thevacuum pump towards a process chamber connected to an inlet of thevacuum pump.

A further vacuum pump, preferably a booster pump, may be providedbetween the isolation valve and the vacuum pump.

A third aspect of the present invention provides a method for inhibitingthe propagation of a flame front ignited by a pumping mechanism drawinga waste stream from a process chamber, the method comprising the stepsof:

-   -   providing a foreline for conveying the waste stream drawn from        the process chamber to the pumping mechanism, the foreline        comprising an isolation valve for selectively isolating the        pumping mechanism from the process chamber and a bypass around        the isolation valve;    -   closing the isolation valve when the waste stream is initially        drawn from the process chamber whereby the waste stream is        conveyed to the pumping mechanism via the bypass, the bypass        comprising means for inhibiting propagation of a flame front        therethrough; and    -   opening the isolation valve when a pressure within a region        upstream of the isolation valve has been reduced below a value        at which propagation of a flame front within the waste stream        can be sustained.

An evacuated region may be generated as the waste stream is drawnthrough the bypass. Consequently, a fourth aspect of the presentinvention provides a method of inhibiting the propagation of a flamefront ignited by a pumping mechanism drawing a waste stream from aprocess chamber, the method comprising the steps of:

-   -   isolating the pumping mechanism from the process chamber using        an isolation valve;    -   providing a bypass around the isolation valve;    -   initially drawing the waste stream through the bypass to        generate within a flow path from the process chamber to the        pumping mechanism an evacuated region that inhibits propagation        of a flame front from the pumping mechanism to the process        chamber; and    -   when a pressure upstream of the isolation valve has been reduced        below a value above which propagation of a flame front within        the waste stream can be sustained, opening the isolation valve.

The evacuated region may be generated between the pumping mechanism andthe isolation valve. The pressure upstream of the isolation valve may bea pressure within a part of the foreline extending between the processchamber and the isolation valve, or it may be a pressure within theprocess chamber. The pressure may be monitored directly and theisolation valve may be opened in dependence on the monitored pressure.Alternatively or in addition to the direct measuring of the pressure,one or more parameters indicative of the transient pressure, for exampletemperature, may be monitored. The pressure may be determined from theone or more monitored parameters and the isolation valve may be openedin dependence on the determined pressure. The isolation valve may beopened after a predetermined period of time. As an alternative tomeasuring the local temperature, one or more of the following parametersmay be monitored to give an indication of the pressure upstream of theisolation valve: pump motor power or current, flow of gas into theprocess chamber or a parameter related to reactions occurring within theprocess chamber (for example plasma reflected power or optical emissionsspectrum).

A further isolation valve may be provided in the bypass. The furtherisolation valve may be closed prior to operation of the pumpingmechanism, and opened when a pressure within a region between thefurther isolation valve and the pumping mechanism has been reduced belowa value at which propagation of a flame front within the waste streamcan be sustained.

The invention is described below in greater detail, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 illustrates a vacuum pumping arrangement comprising a firstembodiment of an apparatus for inhibiting propagation of a flame front;

FIG. 2 illustrates a vacuum pumping arrangement comprising a secondembodiment of an apparatus for inhibiting propagation of a flame front;

FIG. 3 illustrates a vacuum pumping arrangement comprising a thirdembodiment of an apparatus for inhibiting propagation of a flame front;and

FIG. 4 illustrates an alternative vacuum pumping arrangement comprisinga fourth embodiment of an apparatus for inhibiting propagation of aflame front.

FIG. 1 illustrates a vacuum pumping arrangement comprising a firstembodiment of an apparatus 10 for inhibiting the propagation of a flamefront ignited by a pumping mechanism drawing a waste stream from aprocess chamber. The apparatus 10 is connected between a process chamber15 and a vacuum pump 20 comprising a pumping mechanism and forming partof the vacuum pumping arrangement. The apparatus 10 comprises a foreline30 for conveying the waste stream from the process chamber 15 to thevacuum pump 20. A controller 25 is provided in combination with anisolation valve 45, typically a gate valve, located within the foreline30 for selectively arresting fluid flow. The isolation valve 45 isselectively opened and closed in response to signals received from thecontroller 25.

Provision of the valve 45 effectively forms two foreline portions, thesebeing an upstream foreline portion 35 extending between the processchamber 15 and the isolation valve 45, and a downstream foreline portion40 extending between the isolation valve 45 and the vacuum pump 30. Abypass 50 is provided around the isolation valve 45 to provide a flowpath through which the waste stream is drawn from the process chamber 15to the vacuum pump 20 when the isolation valve 45 is closed. One end ofbypass 50 is connected to the upstream foreline portion 35 and the otherend of bypass 50 is connected to the downstream foreline portion 40.

In the embodiment illustrated in FIG. 1, the bypass 50 is of smallercross section than the foreline 30 to restrict the flow therethrough.

In operation, the isolation valve 45 is closed to cause the waste streamto be conveyed through the bypass 50 when the vacuum pump 20 is switchedon. The smaller bore of bypass 50, when compared to the foreline 30,acts to restrict the flow of the waste stream drawn from the processchamber 15 by the vacuum pump 20. As the vacuum pump 20 draws the wastestream from the process chamber 15, the downstream foreline portion 40is evacuated to form a region having a pressure below a particularvalue, P_(crit), at which propagation of a flame front can be sustainedtherethrough, effectively creating a barrier to inhibit any suchpropagation. Thus the initial evacuation of the upstream forelineportion 35 and process chamber 15 is effected whilst maintaining aregion, here the downstream foreline portion 40, within which thepressure is reduced below P_(crit) to inhibit any flame front generatedwithin the vacuum pump 20 from propagating upstream towards the processchamber 15.

Once the pressure within the upstream foreline portion 35 has reducedbelow P_(crit), or even lower if a safety margin is to be incorporated,the isolation valve 45 is opened to establish a direct flow path ofincreased conductance along the foreline 30 for subsequent continuedevacuation of the process chamber 15. By establishing the direct flowpath for the continued evacuation, the desired vacuum within the processchamber 15 is achieved more efficiently in terms of speed of evacuationby the vacuum pump 20.

FIG. 2 illustrates a vacuum pumping arrangement comprising a secondembodiment of the apparatus for inhibiting propagation of a flame front,wherein a further valve 55, for example a variable restriction valve, isprovided within bypass 50 and is controlled by signals output from thecontroller 25.

In operation, prior to switching on the vacuum pump 20, the isolationvalve 45 and the further valve 55 are both closed to completely isolatethe process chamber 15 from the vacuum pump 20. In the event that thevacuum pump contains an accumulation of a flammable gas mixture andmovement of the pumping mechanism within the vacuum pump 20 generates anignition source, for example a spark, any flame front from adeflagration generated within the vacuum pump 20 will not be able topropagate back to the process chamber 15.

When the vacuum pump 20 is switched on, the pressure within thedownstream foreline portion 40 will rapidly be reduced below P_(crit),to form a barrier for inhibiting propagation of any flame front, asdiscussed above. The valve 55 is then opened by a signal output from thecontroller 25 to permit passage of the waste stream from the processchamber 15 through the bypass 50 to effect evacuation of the upstreamforeline portion 35 and the process chamber 15. If the valve 55 isprovided by a variable restriction valve, the flow rate of the wastestream and hence the evacuation of the process chamber can be controlledeither according to a predetermined function, in response torequirements of the process or in response to monitored parameters, asdiscussed below in relation to FIG. 4. The flow rate through valve 55 iscontrolled such that the pressure in the downstream foreline portion 40is maintained below P_(crit) to maintain the barrier for inhibitingpropagation of any potential flame front.

As discussed above in relation to the first embodiment of the apparatus10, evacuation is continued through bypass 50 until the pressure in theupstream foreline portion 35 is reduced below P_(crit) (or below afraction of P_(crit) where a safety factor is introduced). Thecontroller 25 then outputs a signal that causes the isolation valve 45to open so that the conductance of the flow path through which the wastestream is drawn from the process chamber 15 into the vacuum pump 20 isincreased.

The configuration of this second embodiment thereby permits completeisolation of the process chamber 15 from the vacuum pump 20 to beachieved, hence enhanced protection may be provided in circumstanceswhere a more hazardous environment is anticipated.

FIG. 3 illustrates a vacuum pumping arrangement comprising a thirdembodiment of the apparatus for inhibiting propagation of a flame front,in which the bypass 50 comprises a flame arrester element 60. Bypassisolation valves 65 are additionally provided to isolate the bypass 50from the upstream and downstream foreline portions 35, 40 during thepart of the evacuation in which the isolation valve 45 is open. In thisembodiment, the controller 25 is configured to control the actuation ofeach of the valves 45, 65 by outputting respective signals thereto.

In operation, the bypass isolation valves 65 are initially opened andthe isolation valve 45 is initially closed to cause the waste stream tobe initially drawn from the process chamber 15 to the vacuum pump 20through the bypass 50. If a deflagration occurs before the pressure inthe upstream foreline portion 35 reduces below the P_(crit) value, theflame arrester element 60 serves to inhibit propagation of the flamefront towards the process chamber 15. Once the pressure within theupstream foreline portion 35 drops below P_(crit) the isolation valve 45is opened by the controller 25 to permit flow of the waste streamtherethrough.

Whilst in the apparatus of FIG. 3 the bypass 50 is of similar internalcross sectional dimension to the foreline 40 it may be of reduceddimension as discussed above. Furthermore, a further valve 55 canadditionally be provided within the bypass 50 to achieve the associatedbenefits described above.

In practice, depending on the particular processes being undertaken inthe process chamber, the waste stream may comprise condensable or otherdeposit-forming species. These species cause blockage of one or morecomponents placed within the waste stream, for example the flamearrester element 60 and/or the further valve 55. An increased rate ofdeposit formation is likely to occur where these components are placedin the bypass 50 due to the slower flow rates experienced in thisregion. A reduced flow rate is particularly detrimental since it notonly promotes the formation of pockets of increased concentration ofspecies, but also will lack a flushing effect experienced with a higherflow rate. In the event that this type of deposit-forming species arebeing drawn from the process chamber 15 it is beneficial to providebypass valves 65 which can be closed once the isolation valve 45 hasbeen opened. In this way, the bypass 50 is not exposed to the wastestream during the bulk of the evacuation procedure where the isolationvalve 45 is open.

FIG. 4 illustrates a vacuum pumping arrangement additionally having abooster pump 70 located between the isolation valve 45 and the vacuumpump 20. A booster pump 70 is provided to enable the vacuum pumpingarrangement to reduce the pressure within the process chamber 15 belowthat which can be achieved by the vacuum pump 20 acting alone.

Bypass isolation valves 65 are provided as discussed in relation to FIG.3 above. Each valve is controlled in sequence by the controller 25. Oneor more sensors 80, 85, 90 are provided for monitoring either thepressure at one or more locations upstream of the vacuum pump 20 oranother parameter from which the pressure at any particular moment canbe derived, for example temperature. Each sensor 80, 85, 90 outputs tothe controller 25 a signal indicative of the respective monitoredparameter. The controller 25 uses these signals to determine the timingand sequence of the actuation of each of the valves 45, 55, 65 tocontrol the flow of the waste stream.

Potential locations for the upstream sensors 80, 85 are within theupstream foreline portion 35 or within the process chamber 15. Theoutput from one of these sensors 80, 85 is used to determine when thepressure upstream of the isolation valve 45 falls below P_(crit) so thatthe isolation valve 45 can be opened as soon as possible whilst thepropagation of a flame front towards the process chamber 15 isinhibited. In safety critical systems, more than one sensor 80, 85 maybe used to ensure that the system has in-built redundancy. A furthersensor 90 may be provided in the vicinity of the vacuum pump 20 tomonitor either the pressure or another parameter indicative of thepressure in or around one of the vacuum pumps 20, 70. If the pressuremonitored by this sensor 90 rises above P_(crit) indicating a failurewithin the system, the isolation valve 45 can be closed to isolate theprocess chamber 15 from the vacuum pumping arrangement, or other stepsto mitigate a potential deflagration, such as local introduction ofpurge gas or termination of the supply of flammable mixture, can beinitiated. In other words, sensor 90 provides a redundancy within thesystem to allow a fail safe apparatus to be achieved.

1. An apparatus for inhibiting the propagation of a flame front ignited by a pumping mechanism drawing a waste stream from a process chamber, the apparatus comprising: a foreline for conveying the waste stream drawn from the process chamber to the pumping mechanism, the foreline comprising an isolation valve for selectively isolating the pumping mechanism from the process chamber and a bypass around the isolation valve; and a controller for actuating the isolation valve, wherein the controller is configured to: cause the isolation valve to be closed when the waste stream is initially drawn from the process chamber so that the waste stream is conveyed to the pumping mechanism via the bypass, the bypass comprising means for inhibiting propagation of a flame front therethrough; and cause the isolation valve to be opened when a pressure within a region upstream of the isolation valve has been reduced below a value at which propagation of a flame front within the waste stream can be sustained.
 2. The apparatus according to claim 1 wherein an internal dimension of the bypass is smaller than that of the foreline to restrict flow of a waste stream therethrough.
 3. The apparatus according to claim 1 wherein the inhibiting means comprises valve means for effecting a variable restriction to the flow of a waste stream therethrough.
 4. The apparatus according to claim 3 wherein the controller is configured to cause the valve means to be closed prior to operation of the pumping mechanism, and opened when a pressure within a region between the valve means and the pumping mechanism has been reduced below a value at which propagation of a flame front within the waste stream can be sustained.
 5. The apparatus according to claim 4 wherein the controller is configured to prevent initiation of operation of the pumping mechanism unless the valve means is closed.
 6. The apparatus according to claim 1 wherein the inhibiting means comprises a flame arrester element.
 7. The apparatus according to claim 1 wherein the bypass comprises convolutions.
 8. The apparatus according to claim 1 wherein the isolation valve comprises a gate valve.
 9. A vacuum pumping arrangement comprising: a vacuum pump having an inlet; and an apparatus connected to the inlet of the vacuum pump for inhibiting the propagation of a flame front ignited by a pumping mechanism drawing a waste stream from a process chamber, the apparatus comprising: a foreline for conveying the waste stream drawn from the process chamber to the pumping mechanism, the foreline comprising an isolation valve for selectively isolating the pumping mechanism from the process chamber and a bypass around the isolation valve; and a controller for actuating the isolation valve, wherein the controller is configured to: cause the isolation valve to be closed when the waste stream is initially drawn from the process chamber so that the waste stream is conveyed to the pumping mechanism via the bypass, the bypass comprising means for inhibiting propagation of a flame front therethrough; and cause the isolation valve to be opened when a pressure within a region upstream of the isolation valve has been reduced below a value at which propagation of a flame front within the waste stream can be sustained.
 10. The vacuum pumping arrangement according to claim 9 comprising an additional vacuum pump located between the isolation valve and the vacuum pump.
 11. The vacuum pumping arrangement according to claim 10 wherein the additional vacuum pump is a booster pump.
 12. A method for inhibiting the propagation of a flame front ignited by a pumping mechanism drawing a waste stream from a process chamber, the method comprising the steps of: providing a foreline for conveying the waste stream drawn from the process chamber to the pumping mechanism, the foreline comprising an isolation valve for selectively isolating the pumping mechanism from the process chamber and a bypass around the isolation valve; closing the isolation valve when the waste stream is initially drawn from the process chamber so that the waste stream is conveyed to the pumping mechanism via the bypass, the bypass comprising means for inhibiting propagation of a flame front therethrough; and opening the isolation valve when a pressure within a region upstream of the isolation valve has been reduced below a value at which propagation of a flame front within the waste stream can be sustained.
 13. The method according to claim 12 wherein an evacuated region is generated as the waste stream is drawn through the bypass.
 14. A method of inhibiting the propagation of a flame front ignited by a pumping mechanism drawing a waste stream from a process chamber, the method comprising the steps of: isolating the pumping mechanism from the process chamber using an isolation valve; providing a bypass around the isolation valve; initially drawing the waste stream through the bypass to generate within a flow path from the process chamber to the pumping mechanism an evacuated region, that inhibits propagation of a flame front from the pumping mechanism to the process chamber; and opening the isolation valve when a pressure upstream of the isolation valve has been reduced below a value above which propagation of a flame front within the waste stream can be sustained.
 15. The method according to claim 13 wherein the evacuated region is generated between the pumping mechanism and the isolation valve.
 16. The method according to claim 14 wherein the pressure is a pressure within a part of the foreline extending between the process chamber and the isolation valve.
 17. The method according to claim 14 wherein the pressure is a pressure within the process chamber.
 18. The method according to claim 17 comprising monitoring the pressure directly and opening the isolation valve in dependence on the monitored pressure.
 19. The method according to claim 18 comprising the steps of: monitoring a parameter indicative of the pressure; determining the pressure from the monitored parameter; and opening the isolation valve in dependence on the determined pressure.
 20. The method according to claim 19 wherein the isolation valve is opened after a predetermined period of time.
 21. The method according to claim 20 wherein the method comprises the further steps of: providing a further isolation valve in the bypass; closing the further isolation valve prior to operation of the pumping mechanism; and opening the further isolation valve when a pressure within a region between the further isolation valve and the pumping mechanism has been reduced below a value at which propagation of a flame front within the waste stream can be sustained.
 22. The method according to claim 12 comprising monitoring the pressure directly and opening the isolation valve in dependence on the monitored pressure.
 23. The method according to claim 12 comprising the steps of: monitoring a parameter indicative of the pressure; determining the pressure from the monitored parameter; and opening the isolation valve in dependence on the determined pressure.
 24. The method according to claim 12 wherein the isolation valve is opened after a predetermined period of time.
 25. The method according to claim 12 wherein the method comprises the further steps of: providing a further isolation valve in the bypass; closing the further isolation valve prior to operation of the pumping mechanism; and opening the further isolation valve when a pressure within a region between the further isolation valve and the pumping mechanism has been reduced below a value at which propagation of a flame front within the waste stream can be sustained. 